USDA-ARS Northwest Irrigation and Soils Research Laboratory Station Note #02-98.
Prepared by R.E. Sojka, R.D. Lentz, D.L. Bjorneberg, and J.K. Aase, Northwest Irrigation and Soils Research Lab, USDA-Agricultural Research Service, 3793N-3600E, Kimberly, ID 83341
Commercial use of polyacrylamide (PAM) in irrigation water began in 1995 and has been enormously successful and environmentally beneficial. Over 600,000 acres were PAM-treated in the USA in 1997. The overseas market is much larger. The USDA Natural Resources Conservation Service has recognized the Kimberly ARS development of this technology as possibly the most successful soil conservation practice ever developed for furrow irrigation. New uses for the PAM soil conservation technology are being developed at a rapid pace. This pamphlet gives practical considerations for PAM-use in furrow and sprinkler irrigation, drawing on research conducted by the Kimberly ARS team since 1991, and from other published sources.
PAM Material Considerations:
Purchase only polyacrylamide (PAM) officially labeled for irrigation erosion control by your state (37 states currently have agricultural soil fertilizer and amendment laws governing product label/registration). Purchasing from known reputable agri-chemical suppliers may avoid acquiring ineffective or dangerous formulations inappropriate for this use. In the USA this restricts use to PAMs with the following properties:
Purchase PAMs with reasonably high chemical analysis regardless of "type of formulation". PAMs are available in a number of formulation types. The two most common forms are as dry granular beads/powders or as concentrated inverse emulsions. The dry forms of PAM should be expected to be 80% or greater active ingredient (ai) by weight. Inverse emulsions are usually either 30 or 50% ai. In rare instances PAM can be purchased as straight aqueous solutions, however viscosity limits these to 3% ai. PAM is also available as large blocks or cubes. Remember that regardless of form or cost, the amount of PAM purchased is determined by how much active ingredient is in the product. Each PAM form provides ease of application for a specific need, but be sure how much you are paying for PAM active ingredient and how much you are paying for convenience of application. A typical price range for PAM at this writing is $3 to $6 per pound of PAM active ingredient.
Shelf life and storage needs should be considered at the time of purchase. Buy only the PAM needed for a given growing season, as full-potency shelf life for most forms is only about one year. Some loss of effectiveness may be noted when using PAM that is more than a year old. All PAM forms should be stored in cool dry places away from direct sources of sunlight to avoid PAM breakdown and loss of potency. Avoid freezing of liquid forms. Emulsified concentrates may need some mixing before injection or dilution if stored for prolonged periods because PAM and carrier may separate (like oil-base paint). If spills occur, avoid traffic in the spill area, especially if wet, since high concentration PAM solutions are extremely slippery and can cause accidents. Clean thoroughly with sand, sawdust, or the like before attempting to wash down the spill with water.
Note the following use considerations for the various forms of PAM:
Dry PAM forms are easy to store, transport and meter into head ditches but require vigorous agitation to dissolve. Numerous inexpensive metered applicators are available. Dry applicators should be placed on the head ditch immediately above a source of turbulence, and if possible, 100-300 ft before the first irrigated furrow. When applying PAM powder via a turbulent fountain with a weed screen, be sure to apply the PAM below the screen (outflow side) to prevent clogging of screens; a small hole may need to be cut in the screen to allow for this. Powder forms can be easily sprinkled in dry patches at the furrow inflow point (typically 0.5 to 1 ounce per furrow), but care should be taken to avoid over application. Spread the patch along the first 3-5 feet of furrow to avoid burying or loss of PAM by the furrow stream.
Aqueous PAM concentrates accelerate the dissolving of PAM in head ditches, but are very high in viscosity, bulkier than dry forms or inverse emulsion concentrates, have low ai and are now rarely available commercially. Protect against freezing during winter storage.
Emulsified Concentrates (Inverse Emulsions) are flowable liquid concentrates with the appearance and viscosity of cream. The terminology "inverse emulsion" refers to the chemical encapsulation of PAM molecules by minute amounts of mineral spirits, which in turn are covered with a surfactant layer. This allows high concentration (typically 30-50% PAM ai) in liquid form without the high viscosity of aqueous emulsions. When inverse emulsions are dispersed in water, the surfactant strips away the mineral spirit encapsulating layer, releasing the PAM. Emulsified concentrates can provide enhanced dissolution when used in head ditches, but still require moderate mixing at the point of addition to the water stream. Experience has shown that when emulsified concentrates are stored for long periods, some mixing of the concentrate is needed before use to prevent separation of the PAM from the carrier. Emulsified concentrations are bulkier than dry forms, but add convenience of dispensing and mixing. If PAM is to be used in sprinkler irrigation then the emulsified concentrate form of PAM is required.
Blocks or Cubes of PAM are solid PAM forms. Blocks typically weigh 2 pounds. Percent ai is 54.5%. Blocks are suspended in baskets in flowing ditches at turbulent points and slowly dissolve to deliver low concentrations of PAM. Blocks do not perform as well as other PAM forms for furrow irrigation erosion control, but have been useful for dosing settling ponds to promote flocculation and to accelerate water clarification, or to dose concentrated runoff areas on fields that otherwise cause severe uncontrolled erosion. Blocks contain 40% polyethelene glycol (PEG) by weight.
Regardless of form, consider safety needs at all times. READ THE LABEL Properly registered PAMs for use as soil amendments for irrigation induced erosion control will have specific safety instructions for each product. In general, this class of PAMs is relatively safe compared to fungicides, herbicides and insecticides. However, common sense precautions should prevail. Avoid prolonged contact with the skin. If using dry forms, avoid inhaling dust and keep it out of eyes. Rinse exposed areas thoroughly to avoid skin irritation.
Be aware that, although all PAMs are polymers, NOT ALL POLYMERS ARE PAM. The media's recent high interest in PAM and the many stories reporting the effectiveness of PAM have often referred to PAM only as "the polymer." This has encouraged some entrepreneurial exploitation of the word "polymer" in the agri-chemical community. The results achieved with the specific class of PAMs described herein are not known to be achievable with other affordable chemical polymers. Again, purchasing from known reputable agri-chemical supply sources is the best way to avoid problems.
PAM Use Considerations:
Always add PAM slowly to turbulent water; NEVER ADD WATER TO PAM. If water is added to PAM, or if PAM is added to water so fast that wet PAM grains or emulsion drops contact each other before completely dissolving, a gel layer forms around the partially hydrated drops. This essentially stops the remaining PAM from dissolving. Globs (sometimes called "fish eyes" because of their appearance) or gel slabs form that do not dissolve. These globs can clog pipes, dispensers etc. They can be washed away in the water flow without providing any benefit in the field, resulting in under-application because of incomplete dissolving of dispensed PAM. An exception to this rule is use of the PAM patch application method, in which 0.5 to 1 ounce of PAM is sprinkled along the furrow-bottom just below the water source prior to turning in the water. With the patch method, you are intentionally forming a small gel slab at the inlet of the furrow. The slow dissolving of the slab by the furrow stream doses the irrigation water.
PAM stabilizes existing soil structure but cannot improve soil structure. PAM-use is not a substitute for good soil management. If soils are compacted or aggregate structure is degraded, PAM used in irrigation water cannot rehabilitate the soil to an unimpaired condition. A minor exception to this rule of thumb is that PAM promotes formation of a more permeable soil surface than otherwise results during seal formation with untreated water. When suspended sediments in the furrow stream flocculate and settle out, they coat furrow bottoms with the precipitated solids. Seal formation can occur as a result of deposition of particles initially detatched by erosion, or even without erosion, if irrigation water is high in suspended solids because of upstream contamination with sediment-laden return flows. When PAM is used, seals still form, but they are more permeable (because of floccule structure and aggregation of deposited sediment) than in the absence of PAM.
The first drop of water to reach the furrow should already contain the desired amount of dissolved PAM. PAM stabilizes soil to prevent erosion and surface sealing-which improves infiltration. To stabilize the soil in optimal condition, PAM must be delivered before the structure is destroyed by untreated flowing water. Adding PAM to the furrow after untreated water has already begun to flow, usually helps reduce erosion, but it also stabilizes the surface seal created by the untreated water. As a result, PAM cannot improve infiltration as much as it could have done if added immediately.
If using PAM in sprinkler irrigation, pressurize the pipe and be sure water is being delivered before injecting PAM into the flow. This protocol assures that PAM does not build up in sprinkler lines before water enters the pipes (which would violate the caution of not adding water to PAM). The small amount of water sprinkled before beginning PAM injection does not significantly affect soil properties before PAM stabilization is initiated.
If irrigation water is high in sediment, addition of PAM to the flow can cause settling of sediment in the head ditch or in gated pipe. If irrigating with water high in sediment content, the patch application method may be the best approach to avoid siltation of ditches or pipes. Construction of a small pond at the head ditch for PAM mixing can provide a place for sediments to settle without blocking the main reaches of the ditch. If using gated pipe, turn the gates a little more downward to favor washing out sediments. Be sure that siphon tubes are not set so low in the head ditch that they clog if sediment begins collecting. Increasing siphon tube size will usually prevent tube clogging. High sediment loads may require a slight increase of the PAM application rate to compensate for PAM deactivated by flocculation of suspended sediment.
Do not over apply PAM. Research has confirmed that 10 ppm of PAM in advancing water (the NRCS standard application method) provides erosion control in a wide range of circumstances on nearly all soils. The NRCS standard should be used whenever soil is disturbed (first irrigation, following cultivation or traffic, etc.). Lower PAM rates can be used if good soil erosion control is achieved. Excessive application of PAM can lower infiltration rate or suspend solids in water, rather than promoting settling.
When using PAM, increase furrow inflow rates to prevent longer advance time and excessive infiltration at the upper end of the field. PAM partially alleviates surface sealing in furrows. This maintains higher infiltration rates throughout an irrigation than is usual with untreated water. If irrigation practices are not adjusted, PAM use can worsen variability of infiltration from upper to lower field ends. If inflow rates are increased (doubled or tripled) during initial advance, and then reduced to the least sustainable flow once runoff begins, erosion is still greatly reduced and advance times shortened, thereby improving infiltration uniformity.
PAM applicators are available from numerous suppliers for application of all PAM forms. Local PAM material suppliers can usually recommend vendors of several styles of applicators suitable to their products. Good applicators seldom cost more than a few hundred dollars. When considering applicators, look for sturdiness and ability to calibrate application rate to match the water flow. Liquid applicators and injectors should have back-flow protection, and prevent condensation of water vapor in liquid delivery systems. WE DO NOT RECOMMEND applying PAM as a pre-irrigation spray on furrow bottoms using familiar agrichemical spray equipment. This application method has not proven effective, and often risks gumming-up of the spray equipment, since PAM can gel on contact with some concentrated chemical residues left in sprayers, especially if dry wetable powders have been applied with the sprayer.
When using PAM in Sprinklers. Benefits of using PAM with sprinkler irrigation are much less dramatic than with furrow irrigation. Applying 2-4 lb PAM per acre can reduce erosion and increase infiltration during the irrigation under some conditions. However, beneficial effects last for only one or two irrigations. Many questions are still unanswered about applying PAM through sprinkler systems.
It is a good practice to pump crop oil through the injection pump, hose and valves before injecting PAM. Crop oil coats the inside of the equipment which simplifies cleanup and removes any water that may have condensed inside during storage. The injection pump, hose and valves should also be flushed with crop oil immediately after PAM application is complete. Crop oil removes the PAM without problems that are caused by adding water to PAM. Do not try to clean injection equipment with water unless you want to disassemble the equipment and flush each piece with copious amounts of water. Crop oil greatly simplifies equipment clean-up.
ALWAYS have a check valve between the injection hose and the irrigation line to prevent irrigation water from flowing back through the injection line. If an injection pump or hose fails without a check valve, a soupy, jelly mess will occur that is nearly impossible to clean up.
PAM has been shown to greatly improve water quality of irrigation runoff from individualfields and district-wide return flows. Sediment, phosphorus, nitrogen, biological and chemical oxygen demand, pesticide content, weed seed and microorganism content of irrigation return flows have all been shown to be reduced by use of PAM for irrigation induced erosion control. No negative environmental impacts from PAM, or from the small amounts of AMD contained in registered PAMs, used at recommended rates, have ever been demonstrated for surface waters or soil. PAM is and will continue to be a powerful environmental and farming benefit if responsibly used. For more complete and technically detailed information on PAM and its use for irrigation-induced erosion control and for infiltration management, visit the PAM information website at the PAM Research Page.